This invention relates to a antiskid control system and more particularly to a hydraulically actuated antiskid control system.
Conventionally, various antiskid control systems have been proposed wherein a variable volume chamber defined by a piston is provided to cyclically decrease and increase the brake fluid pressure applied to the wheel cylinders thereby releasing a locking condition of the wheels in response to signals generated from a computer; and a valve is also provided to selectively control the fluid comunication between the master cylinder of the braking hydraulic pressure source and the wheel cylinder, and the fluid communication between the variable volume chamber and the wheel cylinders.
In these systems, the variable volume of the chamber is effectively decreased and increased in response to the movement of the piston defining the variable volume chamber. The control valve is also actuated by the movement of the piston. This means that during the initial stage of antiskid control operation, the piston has to be moved to cause the control valve to interrupt fluid communication from the master cylinder to the wheel cylinders. In other words, the piston moves so as to increase the variable volume of the chamber before the fluid communication between the master cylinder and the wheel cylinders is interrupted by the control valve. Therefore, there is delay in the application of the antiskid control in releasing the locked brake. This not only delays the effect of the skid control but results in a large charge in the fluid pressure in the master cylinder. The feeling of control of the vehicle by the operator is thus adversely affected.
It should also be noted that in the prior art, the stroke of the piston has to be relatively lengthy and the piston has to be relatively large in diameter size to effect a satisfactory variation of the desired volume in the chamber. if it is desirable to reduce the diameter of the piston, then the stroke of the piston must be lengthened to satisfy the effective variation of the desired volume. This results in design difficulties and also in construction complexity.
Some of the above-mentioned antiskid control systems further comprise a by-pass passage which directly connects the master cylinder to the wheel cylinders when the antiskid device fails to operate. The bypass passage is normally closed by another valve which is arranged independently of the control valve for interrupting fluid communication from the master cylinder to the wheel cylinders during a locking condition of the wheels. This arrangement also requires complex construction and results in relatively high manufacturing costs.